correct use of join?

[Joseph Paish]

when i run a (large) script i am writing, this is the output i get on the
screen (there are actually several hundred lines of output). note the "nil"
on the first line.

nil
["10/12/2001", "bought", "10000", "abc", "0.100", 0.2345, "17.43"]
["01/02/2005", "sold", "100", "widget2", "0.143", 0.4567, "22.45"]

assuming that each line of the output is called single_record, when i type
single_record.class, i find out that it is an Array. so far, so good.

when i include the line

puts single_record.join(" ")

in the script in order to print single_record out as a space-separated string,
i get the following message :

my_command_prompt$ ruby ~/ruby_dir/program_name.rb
nil
/my_command_prompt/ruby_dir/program_name.rb:377: undefined method `join' for
nil:NilClass (NoMethodError)
from /my_command_prompt/ruby_dir/program_name.rb:239:in `each'
from /my_command_prompt/ruby_dir/program_name.rb:239

when i use irb to do the example on the web page
http://pine.fm/LearnToProgram/?Chapter=07, everything works perfectly.

if single_record is in fact an array (and single_record.class tells me it is),
what am i doing wrong? I have a feeling it has to do with the appearance of
the word "nil" in the first line of output.

thanks

joe

 

[Robert Klemme]

when i run a (large) script i am writing, this is the output i get on
the screen (there are actually several hundred lines of output).
note the "nil" on the first line.

nil
["10/12/2001", "bought", "10000", "abc", "0.100", 0.2345, "17.43"]
["01/02/2005", "sold", "100", "widget2", "0.143", 0.4567, "22.45"]

assuming that each line of the output is called single_record, when i
type single_record.class, i find out that it is an Array. so far, so
good.

when i include the line

puts single_record.join(" ")

in the script in order to print single_record out as a
space-separated string, i get the following message :

my_command_prompt$ ruby ~/ruby_dir/program_name.rb
nil
/my_command_prompt/ruby_dir/program_name.rb:377: undefined method
`join' for nil:NilClass (NoMethodError)
from /my_command_prompt/ruby_dir/program_name.rb:239:in `each'
from /my_command_prompt/ruby_dir/program_name.rb:239

when i use irb to do the example on the web page
http://pine.fm/LearnToProgram/?Chapter=07, everything works perfectly.

if single_record is in fact an array (and single_record.class tells
me it is), what am i doing wrong? I have a feeling it has to do with
the appearance of the word "nil" in the first line of output.

Obviously the first time you access single_record it's nil. And nil
doesn't join. You can do several things:

puts single_record && single_record.join(" ")
single_record and puts single_record.join(" ")
puts single_record.join(" ") if single_record
puts single_record.to_a.join(" ")

and probably a lot more. But maybe it's just a bug that lead to the nil
and it normally cannot happen.

Kind regards

robert

 

[Joseph Paish]

when i run a (large) script i am writing, this is the output i get on
the screen (there are actually several hundred lines of output).
note the "nil" on the first line.

nil
["10/12/2001", "bought", "10000", "abc", "0.100", 0.2345, "17.43"]
["01/02/2005", "sold", "100", "widget2", "0.143", 0.4567, "22.45"]

assuming that each line of the output is called single_record, when i
type single_record.class, i find out that it is an Array. so far, so
good.

when i include the line

puts single_record.join(" ")

in the script in order to print single_record out as a
space-separated string, i get the following message :

my_command_prompt$ ruby ~/ruby_dir/program_name.rb
nil
/my_command_prompt/ruby_dir/program_name.rb:377: undefined method
`join' for nil:NilClass (NoMethodError)
from /my_command_prompt/ruby_dir/program_name.rb:239:in `each'
from /my_command_prompt/ruby_dir/program_name.rb:239

when i use irb to do the example on the web page
http://pine.fm/LearnToProgram/?Chapter=07, everything works perfectly.

if single_record is in fact an array (and single_record.class tells
me it is), what am i doing wrong? I have a feeling it has to do with
the appearance of the word "nil" in the first line of output.

Obviously the first time you access single_record it's nil. And nil
doesn't join. You can do several things:

puts single_record && single_record.join(" ")
single_record and puts single_record.join(" ")
puts single_record.join(" ") if single_record
puts single_record.to_a.join(" ")

and probably a lot more. But maybe it's just a bug that lead to the nil
and it normally cannot happen.

Kind regards

robert

fantastic.

i used this one since it is the clearest to me :

puts single_record.join(" ") if single_record

there is obviously a logic bug somewhere in the script, but getting it running
without errors was the first step.

thanks again

joe

 

[Florian Groß]

assuming that each line of the output is called single_record, when i type
single_record.class, i find out that it is an Array. so far, so good.

when i include the line

puts single_record.join(" ")

in the script in order to print single_record out as a space-separated string,
i get the following message :

my_command_prompt$ ruby ~/ruby_dir/program_name.rb
nil
/my_command_prompt/ruby_dir/program_name.rb:377: undefined method `join' for
nil:NilClass (NoMethodError)
from /my_command_prompt/ruby_dir/program_name.rb:239:in `each'
from /my_command_prompt/ruby_dir/program_name.rb:239

I'd suggest using the ruby-breakpoint library to place assert {
single_record != nil } statements after every statement that alters the
variable in the code. It will automatically open an interactive IRB
shell at that place if the assertion fails. You can leave the assert()
statements in the code -- they can be setup so that the overhead in
non-debug mode is exactly the same as that of calling a method that does
nothing.

It's available at http://ruby-breakpoint.rubyforge.org/ or via "gem
install ruby-breakpoint".